The present invention relates to a wholly aromatic polyester which is excellent in heat resistance and can be produced by an ordinary polymerizer and melt-molded with ease.
A wholly aromatic polyester which is currently on the market contains 4-hydroxybenzoic acid as a main component. However, since the homopolymer of 4-hydroxybenzoic acid has a higher melting point than a decomposition point, a variety of components must be copolymerized with 4-hydroxybenzoic acid to lower the melting point.
A wholly aromatic polyester containing 1,4-phenyhlenedicarboxylic acid, 1,4-dihydroxybenzene, 4,4xe2x80x2-dihydroxybiphenyl or the like as a copolymerizable component has a high melting point of not lower than 350xc2x0 C., and the melting point is too high to melt-process the wholly aromatic polyester by an ordinary apparatus. Further, a variety of methods have been attempted to lower the melting point of such a wholly aromatic polyester having a high melting point to the temperature where the wholly aromatic polyester can be processed by an ordinary melt-processing apparatus. However, these methods have the problem that while the melting point can be lowered to some degree, mechanical strength at high temperatures (temperatures lower than but close to melting point) cannot be maintained.
To solve this problem, a copolyester containing a combination of 2-hydroxy-6-naphthoic acid, a diol component and a dicarboxylic acid component is proposed in JP-A 56-10526. However, this polyester has the problem that it solidifies quickly at the time of cooling, whereby the polymer is liable to solidify at the outlet of a polymerizer. Further, a copolyester containing a combination of 2-hydroxy-6-naphthoic acid, 4-hydroxybenzoic acid, a diol component and a dicarboxylic acid component is proposed in JP-A 55-144024. However, this polyester has problems in relation to heat resistance and melt-processability.
Heat resistance and moldability (melt-processability) are in a relationship of antimony. The higher the heat resistance of a polymer is, the higher molding temperature is required. Therefore, the degradation of the polymer is significant at the time of molding, thereby causing such problems as the formation of blisters in a molded article by the gas produced by the decomposition of the polymer (deformation by blistering), a deterioration in the color of a molded article (formation of streaks) and the susceptibility of a molded article to corrosion due to the gas components produced by a molding machine. A wholly aromatic polyester having high heat resistance and moldability in good balance is not yet provided at the present time.
The present inventors have made intensive studies to solve the above problems and to provide a wholly aromatic polyester which not only has excellent heat resistance but also can be melt-processed easily at low temperatures. As a result, they have found that it is effective for achieving the above objects to combine a 2-hydroxy-6-naphthoic acid unit and a 4-hydroxybenzoic acid unit in a specific limited ratio. The present invention has been completed by this finding.
That is, the present invention is a wholly aromatic polyester which exhibits optical anisotropy in a molten state and which contains, as essential constituents, the constitutional unit (I) in an amount of 40 to 75 mol %, the constitutional unit (II) in an amount of 8.5 to 30 mol %, the constitutional unit (III) in an amount of 8.5 to 30 mol % and the constitutional unit (IV) in an amount of 0.1 to 8 mol % based on the total of all constitutional units, the constitutional units (I) to (IV) being represented by the following formulas: 
wherein Ar1 is 2,6-naphthalene; Ar2 is at least one selected from the group consisting of 1,2-phenylene, 1,3-phenylene and 1,4-phenylene; Ar3 is at least one selected from the group consisting of 1,3-phenylene, 1,4-phenylene and the residue of a compound having at least two phenylene groups connected with each other at the para position; and Ar4 is 1,4-phenylene.